In order to accelerate the answer, I add some details to my request (sry, I don't find any "edit" button to edit my first message).
I've skipped the first panel when opening PSI, and now I'm trying to add an "account" by using "Account configuration".
It seems that theres is two options : or adding an already existing XMPP account to a server, or creating a new account on a server.
But the thing I don't undestand is : what server are you talking about ??
I thought I would user PSI like BorgChat, and be able to communicate throught my PC inside my LAN. And I start to wonder if PSI isn't, in fact, a kind of IRC software forcing us to create a channel somewhere on a distant server. And the thing is I don't want to connect to any external server.
I'm ok to create a "server" in my LAN, but that's all. However, I don't find any documentation to do such a thing. Moreover, I would like to avoid to create a server, because this would force one of my PC to stay powered ON, and I'd rather just being able to communicate throught NETBIOS connexions, like BorgChat.